The general idea of separating particles using a vibrating inclined surface to which a slurry is applied is already known, for example from NRDC's British Patent Specification 1576469. This shows that it is known to suspend from a framework an assembly of vertically-spaced inclined decks to the upper edges of which a feed of the slurry is supplied while the decks are vibrated. A subsequent water wash removes separated ore particles which have collected on the decks.
This patent is merely exemplary of a number of known separators known as "gravity" separators. The common feature of all of these is a vibrating surface or deck. Some devices have provision for a continuous flow of material to be treated, while others operate on a series of cycles. It is particularly (though not exclusively) to devices of the latter class to which the present invention relates.
The standard nomenclature for this type of device is as follows. The separation surface is called a deck; in practice several such decks are often used at once, one above the other. The slurry of material fed to the deck for separation is called the feed. The heavy particle material to be collected as the finished product is called the concentrate; the actual proportion of heavy particles within this concentrate may vary considerably: when the device shown in GB1576469 is used to separate tin particles, the feed might typically consist of 1% tin, whereas the tin content of the concentrate from the device of the present invention might typically be 25% tin. The material collected for reprocessing and/or recirculation is called the middlings. The waste material, containing a low proportion of heavy particles is called the tailings.
Details of the cycling type of separator will now be described. A single cycle of such a separator consists of a feed stage, a wash stage, a middlings flush stage and a concentrate flush stage. A single cycle takes, for example, typically 8 minutes.
During the feed stage which takes approximately 4 minutes, the feed material is fed across the full width of the upper end of the sloping shaken deck so that it flows slowly down over the surface forming a layer of perhaps 2 mm to 5 mm deep. Much of the lighter waste material flows off the lower end of the deck during this period, but perhaps half still remains mixed with the heavier particles.
At the end of the 4 minute period, the feed is turned off and the washing commences. During this stage, wash water is fed from the upper end of the deck, which continues to be shaken. The washing action tends to assist in the separation process of the material on the deck; as the washing continues, the surface at the upper end of the deck largely clears of all the waste material, and leaves behind only the heavy concentrate. This clearance of most of the material continues slowly and progressively down the length of the deck. Thus, as one looks at the deck during the washing process, one can see that waste material is being cleared from the deck starting at its upper end with the edge of the cleared section being distinctly visible as a relatively broad band of change from lesser to greater density which moves slowly down the deck.
It will be appreciated that the upper end of the deck tends to collect little material, but including a relatively high proportion of denser particles. The amount and composition of the material changes progressively down the deck to greater quantities but of lesser concentration of denser particles. The washing stage is stopped after approximately 3 1/2 minutes, after which time it found in practice that the concentrate (that is that portion of the material on the deck which has a sufficiently high concentration of heavy particles) is located in the upper three-quarters of the deck, while the middlings (that is material that is worth recycling) occupies the lower quarter.
During the next stage, the middlings flush stage, the material from the lower quarter of the deck is flushed off the surface with jets of water and is collected for reprocessing or recirculation This typically takes only about 5 to 10 seconds.
Finally, comes the concentrate flushing stage. During this stage, water jets are used to flush the concentrate off the remaining three-quarters of the surface into a collection trough. This takes typically 10 to 15 seconds.
It is important to realise that any separation plant needs to process material continuously, so separators of this kind must be arranged to work in pairs, or paired groups, so that there is no interruption in the flow of feed to a group of separators. Thus, the final three stages, namely the wash stage, the middlings flush stage and the concentrate flush stage must take no longer than 4 minutes, so that the separator is again ready to accept feed material for the next 4 minute period. In this way, one of the separators of the pair can be in the feed stage, while the other carries out the wash, middlings flush and concentrate flush stages.
One of the main disadvantages of the prior art is the very large number of manual adjustments that have hitherto been required Before setting up the apparatus for use the operator may typically have to adjust the tilt of the deck, the speed of oscillation, the stroke amplitude, the feed time and the wash time. The optimum values of these variables will depend upon, amongst other things, the relative densities of the heavier particles and the waste, the particle size distribution in the feed, the flow rate of the feed, and the density of the feed.
Traditionally, the required adjustments have been carried out by skilled personnel who relied merely upon trial and error, and upon experience. It has been clear for many years, however, that the required adjustments were so many, and the opportunities for comparative testing so infrequent, that even skilled personnel found it extremely difficult to obtain optimal or nearly optimal operating conditions, even where the feed rate was constant. In practice, the feed rate to such a separator oft en varies substantially with time, as may the particle distribution density within the feed, and even the particle composition within the feed. Under such circumstances, it has hitherto been difficult to obtain and impossible to maintain optimal operating conditions.
Traditionally, one or more operators in charge of a group or different groupings of parallel (but not necessarily similarly adjusted) machines have assessed how the machines were operating by visual inspection. Constant variations in the quantity and consistency of feed necessitate frequent inspection and adjustment of individual machines if the best performance is to be achieved. This is a function that is often difficult to maintain in practice particularly in the case of separators of the kind where effecting visual inspections is only possible for a short period within the total cycle time.
Finally, known separators have tended to be large and heavy machines, which are difficult and costly to maintain. Running costs have been high, since powerful motors have been required to drive the heavy separating surfaces.
In an entirely different field, difficulties have been experienced in achieving a uniform slurry flow rate across the entire width of the deck. This tends to result in the separation proceeding more rapidly in some places than in others, so making it more difficult to find a single horizontal cut-off line, above which the material on the deck will be collected as concentrate, and below which it will be recycled as middlings.